This invention relates to the field of packaging of microelectronic and microelectro-mechanical systems (MEMS) devices. More particularly, it relates to establishment of a hermetic seal for a cover (such as an optical window) on a package for a microelectronic device such as an optical microelectronic or MEMS product, particularly a large package.
The packaging of micro devices such as microelectronic devices, micromechanical devices and microelectro-mechanical systems is a complex field of endeavor. Within that field, the packaging of microelectro-mechanical systems presents even greater challenges than does the packaging of standard semiconductor (microelectronic) devices such as integrated circuits (ICs). MEMS typically include not only electronic circuitry such as may be found in conventional ICs, but also moving, micro-machined structures such as micromirrors, switches, multiplexers, cross-connects, optical filters and attenuators. In addition to the usual packaging needs of supporting and protecting devices, allowing for electrical interconnection and dissipating heat, MEMS products also require that their packaging permit free movement of the micro-structures, allow interaction of those microstructures with the environment in some fashion appropriate to the particular product, and provide environmental protection for the devices. Typically, a MEMS device is packaged in a housing formed by a first member which establishes a cavity for receiving the device and which has an aperture (through which the MEMS device is inserted) which is closed by a second member that acts as a cap (also called a lid or cover and, in the case of an optical MEMS, a window or optical window). The cavity-forming member often is made of a ceramic material (e.g., alumina) and its coefficients of expansion in the plane of an attachment surface usually are close to those of the materials (typically silicon-based) of which the attached MEMS device is manufactured. By matching those expansion coefficients fairly closely, the shear strength of the adhesive securing the MEMS device to the housing is not exceeded as the temperature of the assembly varies.
Thermal expansion mismatch between the housing material and the cap material also is of concern because such mismatch places stresses on the joint between those two elements. Those stresses may be sufficient to break the attachment between the housing and cap, or to stress the cap to the point it cracks or develops unwanted conditions.
When the MEMS device is optical in nature and operates with an optical input and/or output signal, then the interaction with the environment involves the passage of light into and/or out of the device. The packaging for such an optical MEMS device must permit the ingress/egress of the optical signal(s) with minimal loss and distortion (such as dispersion or undesired refraction). For this purpose, the cap is made of a transparent material, such as a glass, to provide an optical window closing the cavity. Mating and fastening the cap to the cavity housing member is not a simple matter. For example, the MEMS product may require a hermetically sealed environment to keep out humidity and elements that may degrade performance or life, or the MEMS product may need to be kept in a special atmosphere which one does not want to leak out. Typically, such a seal thus must be impervious to gaseous contaminants that could impede the operation of the MEMS device if they leaked into the cavity. The seal between the cavity material and the optical window also must withstand a variety of mechanical stressors, such as vibration and thermal expansion differentials between the cavity housing and the optical window (lid), as well as changes in barometric pressure. The light being processed, in fact, can be one source of thermal energy that the window, the microstructure and other elements may absorb and convert to heat. Temperature cycling, moreover, can be expected to occur due to failures in thermal management systems and energy from nearby components, as well as other causes.
Not only must the seal be capable of withstanding the thermal stresses that may develop in shipping and operation, but also it must not introduce undesirable motion onto the optical window. Motion of the optical window can alter the refraction of the inbound or outbound beam(s) and cause the device to malfunction or at least to perform in an unplanned, undesired or degraded manner. Some motion is inherent due to thermal expansion and other mechanical forces such as barometric pressure changes. Controlling that motion is important. Barometric pressure changes produce forces acting in a z-axis direction (generally normal to the plane of a nominally flat cap), while thermal expansion of the materials produces forces primarily in the x- and y-axis directions (i.e., in or parallel to the plane of the cap). Barometric pressure changes occur not only in use, but also in shipment. It is important that a seal not fail or cause another part of the assembly to fail, and that neither a seal nor any other component be permanently altered in any adverse way during product shipment (e.g., by airplane). With respect to z-direction movement, it is most important at all times that the cap not tilt when it moves along the z-axis (e.g., in response to barometric pressure or internal gas pressure changes).
One approach employed previously is to solder seal a glass cap to the (typically ceramic) cavity housing. The seal material normally is a metal solder which is placed as a buffer between the glass cap and the ceramic cavity rim around the aperture. A temperature of around 330xc2x0 C. most commonly is needed to cause the solder to flow, although there are lower temperature solders. However, the use of a lower temperature solder runs the risk that the seal will fail if the device is subjected to higher temperatures during system assembly or operation. There are also organic seal materials with can be used instead of metal solders. While the latter are often more durable, the seal they form is not hermetic. Consequently, metal solder is needed when the seal must be hermetic.
However, as the size of optical MEMS products increases, such seals no longer suffice, for multiple reasons. One reason is that a solder seal pins the edges of the optical window in place. With barometric pressure changes, the window material therefore has to bow inwardly or outwardly, with maximum flexing at the middle. This flexing introduces strain in the glass window and such glass strain, in turn, introduces errors (i.e., distortion) in optical signal transmission. If the strain gets too high, the glass may crack. The larger the size, the greater the pressure (stress) on the seal joint and on the window material itself due to mismatch of expansion coefficients, as the product temperature and barometric pressure deviate from the conditions under which the product was manufactured. Optical MEMS devices are now contemplated that are or may get to be quite large compared to conventional ICs and prior MEMS products. For example, arrays of optical elements such as mirrors (used in add/drop multiplexers, switchers, attenuators, filters and the like) could, with suitable packaging, easily be five centimeters or more on a side. This large size creates packaging demands of magnitude, if not kind, unknown in the realm of conventional ICs and MEMS packages. The magnitude of stresses on the window and on the window seal are far higher, for the same temperature swing, than much smaller ICs apply to their packaging joints. With such large packages, simple solder seals may fail and caps may crack due to high stress. Also, with such dimensions, small degrees of tilt on the window may seriously impair optical performance.
To prevent the window cracking under the higher forces which will be placed on it, one might wish to increase the thickness of the glass, but that could introduce greater optical distortion, higher losses in the glass and more thermal motion due to the increased bulk. It also would concentrate the stress on the solder joint, raising the failure rate there and/or requiring more robust joints of different construction.
Naturally, an obvious choice is to try to use materials for the cap and the cavity with closely matched coefficients of thermal expansion, to at least minimize stress due to thermal effects (notwithstanding the barometric pressure variation problem, which requires some other solution). However, cost-effective candidate materials are not plentiful in number; neither are they inexpensive. For example, a ceramic package typically will have a thermal expansion coefficient of about 6.9 to 7.2xc3x9710xe2x88x926/xc2x0 C. Sapphire, with a similar thermal expansion coefficient, could be used for the cap but it is quite expensive and has a high index of refraction as well as high transmission loss. Coatings can help reduce the transmission loss for normal incidence but they add cost and do nothing to reduce the refractive index, so both loss and optical path shifts are a concern for non-normal incidence. Other materials that might be considered for the cap (window) might include, for example, borosilicate glasses such as are used in EPROM windows, and zinc borosilicate and barium borosilicate such as are used in LCD""s. Each has its own deficiencies and none is inexpensive.
Thus, a need exists for an improved seal and sealing technique to be used to affix a cover or window to a cavity-forming housing for a MEMS device, particularly for an optical MEMS device. Such a seal or sealing techniques must be hermetic but has to allow a considerable range of motion in the plane of the cavity aperture due to thermal effects and stresses incurred during assembly. It also must not substantially distort the optical signal or degrade the performance of the device in response to thermally-induced or atmospheric-pressure-induced stress, so it must not introduce much motion normal or oblique to the plane of the aperture. That is, it must exhibit torsional and z-axis (i.e., the axis normal to the aperture plane) stiffness. This latter requirement is particularly important with higher index window materials. Finally, it must have minimal impact on optical performance.
These and other needs are addressed, and advantages obtained, with the use of a seal employing a ring element or member between the cavity housing and the window. The ring preferably takes the form of a bellows. That is, it has a flexible portion intermediate two end portions which are used as attachment portions. One of the attachment portions is hermetically sealed, as , for example, by a solder seal, to the housing wall and the other attachment portion is hermetically sealed to the window, which typically is a glass.
According to a first aspect, the invention comprises a MEMS product comprising a housing member forming a cavity; a MEMS device disposed in the cavity of the housing member; a cap member dimensioned and configured to substantially cover the cavity in the housing member; and a sealing member sealingly attached to and between the housing member and the cap member. The sealing member is configured and arranged to be sufficiently resilient to allow motion due to differential thermal expansion between the housing member and the cap member without breaking either sealing attachment and further allowing the cap member to move in response to changes in barometric pressure In one embodiment, the sealing member has a first attachment portion operatively attached by a first hermetic seal to the housing member and a second attachment portion operatively attached to the cap member by a second hermetic seal and the sealing member is impermeable to gas flow, whereby the cavity is hermetically sealed from ambient atmosphere external to the product. In some embodiments, the MEMS device is an optical MEMS device and the cap member is an optical window member. The sealing member may be configured and arranged so that the cap member, when it moves in response to changes in barometric pressure, does not tilt the cap member.
The bellows member may in some embodiments comprise an intermediate portion between the first and second attachment portions, the intermediate portion being constructed and arranged to allow said motion.
According to another aspect, the invention comprises a seal element for use in installing a cap on a housing for a micro-device, the seal element comprising a bellows which, when used, encircles an aperture in the housing and has a first portion for attachment to the housing adjacent the aperture, a second portion for attachment to the cap and a flexible portion intermediate the first and second portions.
In some embodiments, the flexible portion of the seal element is stiffer in permitting movement in a direction normal to a plane of said aperture than in permitting movement parallel to said plane.
In yet another aspect, the invention comprises a seal member for use in a microelectronic product, said product comprising a housing forming a cavity for receiving a semiconductor device and having a wall defining an aperture through which the device is inserted, and a cap for closing the aperture, the seal member comprising a ring having a compliant portion between first and second attachment portions, and configured and arranged to encircle the aperture so that the first attachment portion can be sealingly attached to the wall and the second attachment portion can be sealingly attached to the cap, both sealing attachments being hermetic sealing attachments.
In some embodiments, the ring may be in the form of a metal bellows.